Noninvasive, nuclear imaging techniques can be used to obtain basic and diagnostic information about the physiology and biochemistry of a variety of living subjects including experimental animals, normal humans, and patients. These techniques rely on the use of sophisticated imaging instrumentation which is capable of detecting radiation emitted from radiotracers administered to such living subjects. The information obtained can be reconstructed to provide planar and tomographic images which reveal the distribution of the radiotracer as a function of time. Use of appropriately designed radiotracers can result in images which contain information on structure (low resolution), function, and most importantly, physiology and biochemistry of the subject. Much of this information cannot be obtained by any other means. The radiotracers used in these studies are designed to have defined behaviors in vivo which permit the determination of specific information concerning the physiology or biochemistry of the subject or of the effect that various diseases or drugs have on the physiology or biochemistry of the subject. Currently, radiotracers are available for obtaining useful information concerning such things as cardiac function, myocardial blood flow, lung perfusion, bone density, liver function, kidney function, brain blood flow, regional brain glucose, and oxygen metabolism.
A variety of radiotracers have been proposed for radioimaging including compounds labeled with either positron or gamma emitting nuclides. For imaging, the most commonly used positron emitting radiotracers are .sup.11 C, .sup.18 F, .sup.15 O, and .sup.13 N, all of which are accelerator produced, and have half-lives of 20, 110, 10, and 2 min respectively. Since the half-lives of these radionuclides are so short, it is only feasible to use them at institutions which have an accelerator on site for their production, limiting their use to approximately 25 medical centers in the U.S. and only about 50 throughout the world. Several gamma emitting radiotracers are available which can be used by essentially any hospital in the U.S. and in most hospitals throughout the world. The most widely used of these are .sup.99m Tc (Tc-99m), .sup.201 Tl, .sup.123 I and .sup.131 I. .sup.201 Tl is a monovalent cation which is used for measuring myocardial blood flow. Both .sup.99m Tc and .sup.131 I can be incorporated into a variety of radiotracers and are widely used in most modern hospitals. .sup.99m Tc is generator produced, has a 6 hour half-life, and emits a 140 keV gamma photon which makes this radionuclide nearly ideal for use with current planar and single photon emission computerized tomography (SPECT) cameras. .sup.99m Tc is a transition metal which forms a wide variety of complexes with molecules containing coordinating ligands (e.g. molecules with free thiol, amine, carboxyl or phosphonate functional groups). .sup.99m Tc labeled compounds have been developed for many diagnostic imaging applications, such as functional studies (e.g. cardiac, renal, liver) and perfusion studies (e.g. myocardial, brain, lung, bone).
Diagnostic imaging kits which employ technetium-99m generally comprise several components, i.e. a source of Tc-99m, a ligand, a reducing agent and an antioxidant. The diagnostic agent is generally formed by reacting Tc-99m in an oxidized state with an appropriate ligand in the presence of a reducing agent under conditions which allow formation of a stable complex between Tc-99m in a reduced state (e.g., III, IV or V valence state) and the ligand. The complex should have the desired property of becoming localized in the target organ upon introduction into a patient. Additionally, an antioxidant may be present to suppress the formation of unwanted impurities from the reduction.
To facilitate handling and storage, the aforementioned components of .sup.99m Tc-based imaging kits are generally kept in a freeze-dried state prior to reconstitution. Such lyophilized components may packaged individually or in various combinations as warranted by the specific application. Prior to administration the components of the kit are reconstituted by the addition of sodium pertechnetate in saline and mixing, if separately packaged. The shelf life of lyophilized .sup.99m Tc-based radiopharmaceuticals prior to reconstitution may be as long as 12 to 18 months. Upon reconstitution, however, the shelf life is only 6 hours. Because many hospitals generally make up a single large batch of injection solution, the short post-reconstitution shelf life imposes serious constraints on efficient management of diagnostic procedures.
Such a short post-reconstitution shelf life is due to regulations concerning bacterial growth in the parenteral solutions. Although a wide variety of antibacterial agents are known in the art, very few have been utilized in radiopharmaceutical preparations. This is primarily due to their incompatibility with antioxidants. For example methylparaben and propylparaben are utilized as antibacterial agents in commercially available .sup.99m Tc-based radiopharmaceuticals. Because they are incompatible with the antioxidant employed in the composition, the concentration of .sup.99m Tc complex must be kept low to avoid the formation of undesireable by-products. The .sup.99m Tc-based by-products diminish the quality and resolution of diagnostic imaging. Other bacteriostats, including phenol, thymol, benzyl alcohol, and phenylethyl alcohol, are similarly imcompatible with antioxidants.
Benzalkonium chloride is known as a topical antiinfective, antiseptic and antimicrobial agent. Benzalkonium chloride is bacteriostatic in low and bactericidal in high concentrations. Gram-positive bacteria are more sensitive than gram-negative bacteria. Benzalkonium chloride is used for application to skin and mucous membranes. It is widely used in over-the-counter ophthalmic solutions and in compositions for cleaning and storing contact lenses. It is also used for the sterilization of inanimate articles, such as surgical instruments. For a review of the antimicrobial activity and use of benzalkonium chloride see W. Gump, "Disinfectants and Antiseptics" in Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 7 (Wiley-Interscience, New York, 3rd. ed; 1979) pp. 815-818.
Benzethonium chloride has similar properties, but, due to its low water solubility, it is primarily utilized for cosmetic applications and in antiperspirant and deodorant sticks.
As noted earlier, all current Tc-99m radiopharmaceuticals require the addition of an anti-oxidant and a bacteriostat to extend shelf life post reconstitution. Presently, bacteriostats are seldom used because they interfere with most anti-oxidants and thus the shelf life is limited by regulations to six hours.
Accordingly, an object of the present invention is to provide a bacteriostatic agent for radiopharmaceutical preparations which is compatible with anti-oxidants.
It has now been surprisingly found that radiopharmaceutical compositions can be obtained wherein both bacterial growth and oxidation are minimized. The present invention has met this need by being compatible with current anti-oxidants, allowing the extension of the shelf life up to 24 hours post reconstitution.